Cooling apparatus for infrared detectors

ABSTRACT

A variable temperature dewar in which a heat load is thermally connected to a heat sink by a short heat pipe, the temperature of the load being varied by varying the pressure in the heat pipe to vary the boiling temperature of a cryogenic fluid contained therein, of which the liquid phase is in conductive thermal contact with the load and the gas phase is in conductive thermal contact with the sink.

United States Patent Bruno et a1.

[ Sept. 17, 1974 COOLING APPARATUS FOR INFRARED DETECTORS Inventors: Robert Peter Bruno, Watertown;

Albert Winston Naugler, Wayland, both of Mass.

Assignee: Honeywell Inc., Minneapolis, Minn.

Filed: Dec. 22, 1971 Appl. No.: 210,668

US. Cl. 250/352, 62/45 Int. Cl G01j 3/02, Fl7c 7/00 Field of Search 250/833 H, 352; 62/47,

62/45 X, 45, 132, 514 A, DIG. 94, DIG. 12

References Cited UNITED STATES PATENTS 10/1960 Cornelison et a1 165/105 Ure, Jr. et a1 250/833 l-l 3,180,989 4/1965 Hand, Jr. et a1. 62/514 3,258,602 6/1966 Promish 62/514 3,369,370 2/1968 Todd, Jr, et a1. 62/514 3,424,230 1/1969 Wright, Jr 62/514 3,512,582 5/1970 Chu et a1. 165/105 Primary Examiner-Archie R. Borchelt Attorney, Agent, or Firm-Charles J. Ungemach; Albin Medved; John S. Munday [5 7] ABSTRACT A variable temperature dewar in which a heat load is thermally connected to a heat sink by a short heat pipe, the temperature of the load. being varied by varying the pressure in the heat pipe to vary the boiling temperature of a cryogenic fluid contained therein, of which the liquid phase is in conductive thermal contact with the load and the gas phase is in conductive thermal contact with the sink.

2 Claims, 1 Drawing Figure PAIENTEDSEP 1 7 m4 ROBERT P. snurvo ALBERT W'NAUGLER INVENT R.

ATTORNEY 1 COOLING APPARATUS FOR INFRARED DETECTORS FIELD OF THE INVENTION This invention relates to the field of cooling apparatus, and specifically to a variable temperature dewar of new and improved type for use with infrared detectors.

BACKGROUND or THE INVENTION In working with such detectors it is relatively easy to attain a singleminirnum temperature, which is that resulting from direct thermal contact between the detector to be cooled and the dewar of liquid cryogen. To

obtain a. temperature which can be varied at will throughout useful range extending upward from the minimum temperature, without incurring the comparable expense of a temperature-variable mechanical refrigerator, is quite a different problem, and one which has not heretofore been solved with complete satisfaction. It is known to vary the rate at which heat may flow to the sink, by mechanically changing the length or cross-sectional area of the heat conductive path, but such arrangements are not easily resettable to a previous adjustment, and in any event have involved manipulation of the dewar of liquid cryogen which is fraught with certain hazards to the operator. It is also known to build into the detector support an electric heater of such capacity that the thermal gradient between the detector and the heat sink may be modified to decrease the rate of heat conduction from the detector and hence allow its temperature to rise above the minimum value: this method is wasteful of cryogenic fluid, and may even require the addition of so much accessory heat as to cause violent boiling of the cryogenic'fluid with consequent danger to the operator.

SUMMARY OF THE INVENTION The present invention solves the problems of varying detector temperatures by using a heat pipe to connovelty which characterize our invention are pointed duct heat from the heat load to the heat sink, and varying the conductivity of the pipe in-a new and convenient manner. The heat pipe is constructed in a form of a pressure chamber to contain cryogenic fluid which vaporizes to extract heat from the load and condenses to transfer the heat to the sink: by varying the pressure in the chamber over a range of say ten atmospheres, the boiling temperature of the liquid in the chamber, and hence the temperature of a detector to be cooled, may be varied. The arrangement is simple, repeatable, and adjustable at a safe distance from the dewar itself.

It is accordingly an object of the invention to provide an improved variable temperature dewar. Another object is to provide such a dewar including. a heat pipe which is adjustable in a novel fashion to vary the temperature of a load at one end thereof by means remote from the dewar. A further object of the invention is to i provide an improved remotely adjustable heat pipe for use in dewars and other applications.

out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and objects attained by its use, reference should be had to the drawing which forms a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING STRUCTURE or THE PREFERRED EMBODIMENT The dewar is shown to comprisea disc 11 of Kovar or other material having a coefficient of thermal expansion compatible with that of glass. Disc 11 has a large central aperture 12 to the rim 13 of which is sealed a reentrant container 14 of glass having an outer wall 15, an inner wall 16 spaced therefrom by arim 17, and a bottom 20.

A cap 21 of glass or other suitable material is secured to the perimeter of disc 11, on the opposite side from container 14, by a suitable vacuum-tight seal and may if necessary have a central aperture 22 closed by a window 23 of material transparent to radiation of a wave length of interest, indicated by the arrow 27. Disc 11 is provided with a tubulation 24, :including a valve 25 and a vent 26, by means of which the space within cup 21 and between the walls of container 14 may be evacuated and sealed.

A heat pipe comprising an inverted cup 31 of stainless steel has one end 32 secured by suitable means in good heat conducting relation to the bottom 20 of container 14, the-other end being securely closed in gas tight fashion by a plug 33 of copper or some other good heat conductor. To the outer surface of plug 33 is secured, in good heat conducting relation, infrared radia 'tion responsive means comprising a detector or detector array generally indicated at 34 and having a suitable plurality of conductive leads 35 formaking electrical connection thereto. The arrangement is such that radiation responsive means 34 is in line with and very close to window 23, to receive radiation 27 transmitter therethrough.

It is desirable to determine the temperature of member 34 directly, and to this end a thermocouple 36 or other suitable device having electric leads 37, 38 is secured in heat conducting relation to plug 33 near member 34. The rim of cup 31 also carries an electric heater 39 having leads 40 and 41, and a multiconductor header 42 is provided indisc 11 for making external connection to leads 37, 38, 40 and 41. A similar header 43 may be provided in disc 11 for making external connection to leads 35 of member 34.

A length of pressure tubing 44 having an expansion loop 45 is soldered or otherwise secured in an opening 46 in disc 11, and in an opening 47 in cup 3l, providing a passage from the interior of cup 31 to a three way valve 50 having a closed position in which connection with the interior of the cup is cut off, a first open position in which the interior of the cup is connected through a tube 51 and vacuum pumping equipment 52 to a vent 53, and a second open position in which the interior ofthe cup is connected through a tube 54, a pressure regulator 55 and a tube 56 to a high pressure cylinder 57 of gas such as oxygen, nitrogen, or argon. In some cases it may be desirable to use a selected mix ture of gases rather than a single pure gas. A suitable gage 60 indicates the pressure in tubing 44, and hence in heat pipe 30.

The FIGURE shows that container 14 is partly full of cryogenic fluid, and is open to the atmosphere, while heat pipe 30 is partly full of fluid but is not open to the atmosphere. Thus the pressure above the liquid in the heat pipe may be made greater than that of the atmosphere.

OPERATION OF THE PREFERRED EMBODIMENT The operation of the invention in infrared technology is as follows. Cap 21 is separated from disc 1 1 and a detector or detector array 34 of interest is secured to copper plug 33, with its various leads connected to conductors in header 43. Cap 21 is resealed to disc 11 and the interior of the dewar is evacuated. A cryogenic material in liquid form, such as liquid nitrogen (N )-is carefully poured into container 14 where it initially boils vigorously into the atmosphere, reducing the temperature of the container and, by conduction, that of heat pipe 30.

The temperature at which liquid nitrogen boils under one atmosphere if pressure is 77K and as is well known the temperature of a liquid cannot rise above its boiling point. The bottom of container 14, and hence the end 32 of heat pipe 30, are thus maintained substantially at 77K as long as there is liquid remaining in the container. Because of the construction of dewar 14, with its double walls and evacuated space therebetween, the main heat input to the liquid in the container is by contact with the air above the surface of the liquid, and after the initial cooling of the glass takes place the liquid boils quietly at the surface layer, the evaporation maintaining the mass of liquid at its boiling temperature.

Valve 50 may now be turned so as to connect the interior of the heat pump with vent 53 through vacuum pumping equipment 52, and the air is removed from cup 31 and tubing 44, after which valve 50 is closed to isolate the heat pipe. When a condition of equilibrium reached valve 50 is turned so as to admit cryogenic fluid into the heat pipe from cylinder 57 under the control of pressure regulator 55. This fluid is preferrably the same as the material in container 14, and a portion of the fluid from the cylinder condenses in the chilled heat pipe and settles as liquid on the upper surface of plug 33. The plug then comes to the temperature of this liquid, and with it the radiation respective means 34. When a desired pressure, say one atmosphere, is indicated by gage 60, the flow of fluid from cylinder 57 is cut off by valve 50. Radiation of interest is now allowed to pass through window 23 and fall upon member 34.

The radiant energy passing through window 23 and falling on detector or detectors 34 is largely absorbed therein, thus adding heat to the lower end of the heat pipe. The temperature of the liquid cannot rise while the pressure in the heat pipe remains constant, but some of the liquid is vaporized and the vapor condenses at the other end 32 of the pipe, thus transferring heat from the detecting means as a load to the fluid in container 14, which acts as a heat sink. The amount of heat reaching the liquid in pipe 30 from all sources including the incoming radiation 27 is not large, and the boiling or vaporizing in the pipe goes on quietly. Suitable indicating means input may be connected to conductors 37 and 38 to display the temperature of the detecting means which will be very close to 77K.

When it is desirable to operate the detecting means at a temperature higher than 77K, additional gas is admitted into the heat pipe by briefly opening valve 50. The pressure in the pipe rises, and with it the boiling temperature of the liquid contained at the bottom of the pipe. Condensation can still occur at the end 32 of the pipe since the temperature is maintained low by the liquid in container 14, but the rise in temperature permitted the fluid in the pipeby the pressure rise also allows the temperature of the detecting means to rise. With nitrogen as the cryogenic material it has been found possible that by allowing the pressure in the pipe to vary from one to ten atmospheres, the temperature of the detecting means may be repeatably and accurately varied between 77K and about K. If argon is used instead of nitrogen the range is from 90K to K.

The purpose of heater 39 is to make it possible to continue raising the temperature of the detector means beyond that feasible by increasing the pressure in the heat pipe. When this is desirable, fluid is removed from the heat pipe, so that the only cooling of the detecting means is by conduction through the stainless steel cup, which has relatively poor heat conductivity so that the detecting means is considerably warmer than end 32 of the pipe. Further temperature increase is accomplished, in conventional fashion, by adding increasing amounts of heat to the pipe by varibly energizing heater 39.

Numerous objects and advantages of our invention have been set forth in the foregoing description, together with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and changes may be made in detail especially in matters of shape, size, and arrangement of parts, within the principle of the invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

We claim as our invention:

1. Radiant energy detecting means to be cooled to a variable temperature below ambient;

a heat sink;

a heat pipe extending between said detecting means and said sink and containing a heat transfer medium adapted for condensation by loss of heat to said sink and for vaporization by gain of heat by said detecting means;

an evacuated chamber enclosing said pipe and said detector means and having at least a portion which is transparent to radiant energy to which said detecting means is sensitive,

and means for varying the pressure in said heat pipe, whereby to vary the boiling point of said medium and hence the temperature of said detecting means.

2. In combination:

a dewar including an evacuated chamber having a reentrant first portion configured to comprise a double-walled container for cryogenic liquid and a second portion transparent to radiant energy in a band of wave lengths and spaced from said first portion;

a heat pipe having first end secured in good heat conducting relation to said container for cooling by said cryogenic liquid, and a second end positioned adjacent said transparent portion;

radiant energy detecting means mounted on said second end of said heat pipe, in good heat conducting relation therewith, for receiving radiant energy enmeans. 

1. Radiant energy detecting means to be cooled to a variable temperature below ambient; a heat sink; a heat pipe extending between said detecting means and said sink and containing a heat transfer medium adapted for condensation by loss of heat to said sink and for vaporization by gain of heat by said detecting means; an evacuated chamber enclosing said pipe and said detector means and having at least a portion which is transparent to radiant energy to which said detecting means is sensitive, and means for varying the pressure in said heat pipe, whereby to vary the boiling point of said medium and hence the temperature of said detecting means.
 2. In combination: a dewar including an evacuated chamber having a reentrant first portion configured to comprise a double-walled container for cryogenic liquid and a second portion transparent to radiant energy in a band of wave lengths and spaced from said first portion; a heat pipe having first end secured in good heat conducting relation to said container for cooling by said cryogenic liquid, and a second end positioned adjacent said transparent portion; radiant energy detecting means mounted on said second end of said heat pipe, in good heat conducting relation therewith, for receiving radiant energy entering said device through said transparent portion thereof; means for supplying to and maintaining in said heat pipe a quantity of cryogenic material at a pressure which is variable through a range of from about one to about ten atmospheres; and means extending through said container for maintaining electrical connection to said detecting means. 